Many electronic devices include acoustic output transducers, i.e. transducers for converting a suitable electrical driving signal into an acoustic output such as a sonic pressure wave or mechanical vibration. For instance many devices may include one or more loudspeakers for sound generation, e.g. for playback of audio content or voice communications and/or providing audible notifications. Some devices may also include transducers capable of generating ultrasonic waves, e.g. for proximity detection type applications and/or machine-to-machine communication. Many electronic devices may additionally or alternatively include haptic transducers for generating vibrations, e.g. for haptic control feedback or notifications to a user. Additionally or alternatively an electronic device may have a connector for making a removable mating connection with a corresponding connector of an accessory apparatus and may be arranged to provide a driving signal to the connector so as to drive a transducer, of one or more of the types mentioned above, of the accessory apparatus when connected.
Such an electronic device will thus comprise driving circuitry for driving the transducer of the host device or connected accessory with a suitable driving signal. For acoustic transducers the driving signal will generally be an analogue time varying voltage signal, i.e. some time varying waveform.
Especially for electronic devices that may be operable in a battery powered mode, e.g. portable electronic devices, power consumption may be a concern and relatively low power consumption may be desirable. For this reason some transducer driving circuits may be implemented using a Class D amplifier.
FIG. 1 illustrates one example of a conventional signal path for driving a transducer using a Class D amplifier. An input signal SIN1 is received and input to a PWM modulator block 101. The PWM modulator block 101 also receives a reference signal SREF and generates a PWM signal SPWM1 with a duty cycle that depends on the level of the input signal SIN1. The PWM signal SPWM1 is used to control switching of an output power stage 102 to generate an output signal SOUT1, which may, in some instances, effectively be an amplified version of the PWM signal SPWM1. The output signal is filtered by a low-pass filter 103 to generate an analogue driving signal SD1 which is supplied to drive the transducer 104.
In some instances the input signal SIN may be compared, by the PWM modulator block 101, to the reference signal SREF which comprises a cyclic reference waveform. In other instances however the PWM modulator block 101 may receive a feedback signal SFB1, as illustrated by the dashed line, that is indicative of the output signal SOUT1 (or in some instances the driving signal SD1) and compare an error between the input signal SIN and the feedback signal SFB1 to the reference SREF1.
Typically the PWM modulator block 101 and output stage 102 may be formed as part of an integrated circuit (IC). The filter 103 may, in some instances, be formed at least partly by components external to the integrated circuit, i.e. off-chip. The transducer 104 may be a transducer of the host device that houses the rest of the signal path, but in some instances the transducer 104 could be transducer of an accessory apparatus that is removably connected via some suitable connector, e.g. a plug-and-socket arrangement or some other plug-and-receptacle connector, e.g. USB, USB-C, Lightning connector etc.
Whilst such transducer driving circuitry can be used satisfactorily in many applications, there is a general desire for ever smaller and/or lower power consumption circuitry, especially in applications where relatively high quality signal reproduction may not be required.